Photoacoustic tomography (PAT) for determining an optical property distribution of a subject such as a tissue with a high resolution by making use of a characteristic of an acoustic wave whereby less scattering occurs in the subject than with light has been proposed in recent years (see NPL1). When the tissue is irradiated with pulsed light from a light source, the light propagates while diffusing through the tissue. A light absorber contained in the tissue absorbs energy from the propagating pulsed light, and as a result, an acoustic wave (typically an ultrasonic wave) is generated (this phenomenon may be referred to as a photoacoustic effect). When the acoustic wave is detected (received) by an acoustic wave detector such as an ultrasonic probe, an acoustic wave signal serving as an electric signal is obtained. By analyzing the acoustic wave signal, an optical property distribution in the tissue, and in particular an optical energy absorption density distribution, can be obtained.
According to NPL1, a sound pressure (P) of an ultrasonic wave obtained from a light absorber in a tissue in PAT can be expressed by the following equation.P=Γ·μa·Φ  [Math. 1]
Γ is a Grüneisen coefficient serving as an elasticity property, which is obtained by dividing the product of the square of a coefficient of cubic expansion (β) and acoustic velocity (c) by a specific heat (Cp) (Γ=βc2/Cp).
μa is an absorption coefficient of the light absorber.
Φ is a local fluence (an amount of light entering the light absorber) in a local region.
The sound pressure of the acoustic wave obtained during PAT is commensurate with the local fluence reaching the light absorber. The light entering the tissue attenuates rapidly within the tissue due to scattering and absorption, and therefore the sound pressure of the acoustic wave generated in deep tissue in the tissue attenuates greatly in accordance with a distance from a light irradiation site.
A backward detection method in which the acoustic wave is detected by emitting light from the same side as the detector and a forward detection method in which the acoustic wave is detected by emitting light from the opposite side to the detector are known as acoustic wave detection methods. In backward detection type PAT, a method in which light is emitted diagonally from a flank of an acoustic wave detector so that the light is emitted effectively onto the back of the acoustic wave detector has been proposed (see NPL2). A main object of this literature is to irradiate a specific site of a tissue with light by disposing a mirror or a lens on the flank of the acoustic wave detector.
When backscattered light on the surface of the tissue enters the ultrasonic wave detector in backward detection type PAT, an ultrasonic wave generated by this light on a surface of the ultrasonic wave detector causes noise. To suppress this noise, a metal film (an aluminum coated plastic film) for reflecting scattered light is provided in front of the acoustic wave detector in NPL2.
Further, PTL1 discloses an example of an ultrasonic probe used in an ultrasonic diagnosis apparatus for obtaining an ultrasonic echo image, in which a metal having surface irregularities is provided on a front surface of the ultrasonic probe as an impedance matching layer. In this example, an average thickness of the impedance matching layer is preferably set at ¼ of a wavelength of the ultrasonic wave and a thickness of the irregularities is preferably set at +⅛, −⅛ of the wavelength of the ultrasonic wave. Note, however, that the metal layer is provided with the aim of widening a bandwidth of the ultrasonic probe and not reflecting scattered light.